Muwatta Malik, Book 17, Hadith 55 (USC-MSA) ⓘ

Yahya related to me from Malik from Nafi that Abdullah ibn Umar would always pay the zakat al-fitr in dates, except once, when he paid it in barley.

وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ نَافِعٍ، أَنَّ عَبْدَ اللَّهِ بْنَ عُمَرَ، كَانَ لاَ يُخْرِجُ فِي زَكَاةِ الْفِطْرِ إِلاَّ التَّمْرَ إِلاَّ مَرَّةً وَاحِدَةً فَإِنَّهُ أَخْرَجَ شَعِيرًا ‏.‏

Maliksaid, "Payment of all types of kaffara, of zakat al-fitr and of the zakat on grains for which a tenth or a twentieth is due, is made using the smaller mudd, which is the mudd of the Prophet, may Allah bless him and grant him peace, except in the case of dhihar divorce, when the kaffara is paid using the mudd of Hisham, which is the larger mudd."